I do not know a lot about the Peter Coffee line. There are a number of current researchers, but I have not seen any definitive work that they have produced. The following information comes from what few files I have on this family. Some of it comes from my recollection of various books that I have read concerning Andrew Jackson and the 1814 Battle of New Orleans.
From time to time a question will appear in one of the many forums that I visit, questioning whether or not there were two Generals John Coffee. There were, and they were Peter's grandsons.
Peter's son Joshua [Jan. 26, 1745-Sep. 8, 1797] married Elizabeth Graves, she probably of the same Graves line into which Edward Coffey married. They had at least five children with number four, John [Jun. 2, 1772-Jul. 7, 1833] becoming a friend, business partner and General who served under Andrew Jackson.
John raised a regiment of volunteers to help Jackson at the Battle of New Orleans in Dec., 1814. After that battle he was promoted from Colonel to Major General, and in 1817 was appointed Surveyor-General of Alabama. He moved to Huntsville in that state and in 1819 moved to Florence in Lauderdale Co. He died at the family home, "Hickory Hill" north of Florence. His wife Mary Donelson, as I seem to recall, was a niece of Rachel Jackson.
Peter Coffee, Jr., the eighth child of Peter and Susannah, married Sarah Smith Nov. 14, 1773 in Granville Co., NC. They had at least nine children, with number five, John E. becoming the second General Coffee.
This General Coffee married Ann Penelope Bryan [Sep. 23, 1784-Dec. 12, 1865]. A history of Dodge Co., GA1 relates that General Coffee, a native of Virginia, was among the first settlers in Telfair Co., GA. He fought in the War of 1812-1815, and represented Telfair Co. in the state legislature. He was also elected three times to the US Congress from Georgia, with the third term  coming before his death had been reported.
Please write to me if you can add to or correct any of this information.
1 History of Dodge County [GA], Cobb, Addiie Davis : Atlanta, Foote & Davies Co., 1932, pgs 271